Municipal and industrial waste water treatment includes an activated sludge treatement process and can include a further anaerobic sludge treatment process depending upon the target conditions with respect to the quality and quantity of the waste water effluents from the waste treatment plant. One of the main goals of these processes is the reduction of aqueous sewage sludge volume. Thus, the cost of dewatering anaerobically digested sludges that contain high volumes of water is becoming the critical economic factor in the overall waste treatment operation.
The dewatering of sewage sludges is one of the most difficult processes to accomplish in sewage treatments. The main reason is that sludges have varying chemical and physical characteristics resulting from the many kinds of organisms which can be present, continuous changes of influent composition, and the variability between treatment plant equipment. For the purpose of sludge dewatering, the anionic nature of digested sludge has been employed to flocculate the sludge with cationic chemical polyelectrolytes. The solid floc formed in this way is then separated from the liquid phase by mechanical dewatering devices, including vacuum filters, centrifugal dehydrators, belt presses, filter presses, and screw presses. Centrifugation is used quite frequently by large municipal waste treatment plants, because it can handle large volumes of sludge on a continuous basis. Thus, many attempts have been made to improve the efficiency of dewatering via centrifugation. A major problem associated with centrifugal separation is that the initial floc strength is reduced as the floc is subjected to shear stresses in the pumps and centrifuges. This floc deformation results in a low yield of sludge solids capture and a high level of resuspended materials in the centrate.
Chitosan, a polyanionic carbohydrate polymer, has been found to be effective for coagulating protein from food product wastes such as brewery and vegetable activated sludge, and its use as a flocculating agent for the treatment of waste water has been reported in Japan. See V. J. McGahren et al, Process Biochem., 19, 88 (1984). However, its use for the dewatering of sewage sludge has not found wide acceptance, possibly due to the relatively low strength of the resultant floc. Thus, a substantial need exists for an improved method to dewater aqueous sludge. A further need exists for a method to treat digested or undigested sewage sludge which yields a floc of high strength which settles and releases water rapidly.